MAE SOT, Thailand — A deadly form of malaria has developed a resistance to the most powerful drugs used to treat the disease, putting the lives of millions of people around the world at risk.

Tests by a team of British and Thai scientists over a 10-year period found the most dangerous species of malaria parasites, spread by mosquitoes, are becoming more resistant to the most effective treatments containing artemisinin, a drug derived from the sweet wormwood shrub.

They discovered that Plasmodium falciparum, which was first reported in 2009 in western Cambodia, is now being found 500 miles (805 kilometers) away on the border of Thailand and Burma.

The details of their findings and research, published in The Lancet medical journal, showed that between 2001 and 2010, the average time taken to reduce the number of parasites in the blood by half following treatment rose from 2.6 hours to 3.7 hours.

The proportion of slow-clearing infections increased during the same period from six to 200 out of every 1,000 cases.

Study leader Professor Francois Nosten, director of Thailand’s Shoklo Malaria Research Unit, warned of a “race against time” to halt the spread of the potentially untreatable malaria.

“We have now seen the emergence of malaria resistant to our best drugs, and these resistant parasites are not confined to western Cambodia,” he said. “This is very worrying indeed and suggests that we are in a race against time to control malaria in these regions before drug resistance worsens and develops and spreads further.”

He added, “The effect of that happening could be devastating. Malaria already kills hundreds of thousands of people a year — if our drugs become ineffective, this figure will rise dramatically.”

In 2010, malaria accounted for the deaths of an estimated 655,000 people across the world, with the victims mainly young children and pregnant women.